The UPS cargo plane that crashed and burned near the Birmingham, Ala., airport this morning never sent a distress signal or message to air traffic control before going down, authorities said today.

The plane was nearing the runway at Birmingham-Shuttlesworth International Airport when it went down around 6:10 a.m., authorities said. It crashed about a half-mile from the airport, hitting the ground three times before skidding to a halt and bursting into flames as it hit the ground.

Both the pilot and co-pilot were killed.

There were no other individuals on board the plane, which erupted into flames as it crashed and its fuel caught fire near the plane's tail, according to Birmingham Mayor William Bell. Bell was briefed by National Transportation Security Board investigators this afternoon.

WHAT TO KNOW

UPS Cargo plane crashed outside of Birmingham airport this morning.

Pilot and co-pilot died in crash.

"We were told that there was no distress signal emitted from the airplane itself, and there were no calls for the airport or the control tower to assume that they were in any trouble. At this point and time that's the information that we have," Bell said.

The plane had taken off from Louisville and was on final approach to Birmingham when it went down, according to authorities.

Robert Sumwalt, an NTSB board member on the investigative team at the crash site, said that authorities were still working to recover the black boxes, or flight data recorder and cockpit voice recorder, from the plane. The team will collect those and other evidence to help determine the cause of the crash.

"The tail section is still smoldering, still smoking and, for that reason, we have not been able to get in and get black boxes, if you will, the flight data and cockput voice recorders," he said at a news conference today. "The firefighters are out there still working to do that, so we are optimistic we will be able to get in there quickly and recover those recorders."

The NTSB responded to the crash site with a 26-member investigative "go team," he said.

Bell said there were UPS packages and some U.S. mail on board the flight when it went down. Rescue workers were still trying to recover the bodies from the plane this afternoon, he said.

Sumwalt said that he would not be able to speculate on the cause or details of the crash while the team investigated.

The plane, an Airbus A300, was manufactured in 2004. The plane was said to be carrying a mixture of heavy cargo and freight and small packages.

It was unclear what caused the crash. Near 6 a.m., the airport had visibility of 10 miles and a cloud ceiling of 700 feet.

"It appears the aircraft went through trees before it impacted at bottom of hill, where a fireball [erupted]," Sumwalt said. "Then, it when up hill and came to final resting point."

"This incident is very unfortunate," said UPS Airlines President Mitch Nichols, "and our thoughts and prayers are with those involved.

"We place the utmost value on the safety of our employees, our customers and the public," Nichols added. "We will immediately engage with the National Transportation Safety Board's investigation, and we will work exhaustively on response efforts."

The company said that family members seeking information on the crash should telephone 800-631-0604.

Retired U.S. Marine Col. Steve Ganyard, an ABC News aviation consultant, said this morning that the NTSB would be able to quickly go into the crash site and begin recovering evidence that would help them understand the cause of the crash, including the way the plane hit the ground and the information recorded by the black boxes on board.

"The one thing that is in favor of getting this resolved quickly is we've got tapes, radar tapes, tower tapes, it looks like it's not that bad of a crash that the black boxes should survive," he said.

"The crash occurred on the runway or in the descent into the airport, so it's going to be controlled. It didn't land in trees on a mountain with terrible destruction. The flat area will allow investigators to do a thorough analysis," Ganyard said. "They can look at where the engines were, whether it was turning or not, what the speed was, if the gear was down, if the control surfaces were operating properly."

The investigators will first try to determine whether anything was specifically wrong with the airplane itself and whether other models of the airplane could pose a danger.

The A300 is one of the most widely-flown aircraft in the world. It was produced from 1974 to 2007 and 561 of the aircraft were delivered.